I am about to pour the insulation layer for my firebrick to sit on and plan on using the 5 to 1 ratio of vermiculite and portland cement. I will filling an area of about 52 by 52 inches and 5 inches deep.

How much vermiculite should I buy? I only want to make one trip to the store.

Based on your measurements, I come up with about 10 cubic feet of Vermiculite. I didn't figure in the volume of portland because it's pretty insignificant.

Buy an extra bag. You don't want to be in the middle of mixing and run out. You can save it for insulating the dome or return it to the store. As you build your oven you'll find yourself making a LOT of extra trips to the store

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.Updated!

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. ... To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts. ... To view links or images in signatures your post count must be 0 or greater. You currently have 0 posts.

It tends to "shrink" or compress a bit when you put it in or if you mix it too aggressively. (You want to mix gently, folding the cement in, to keep the "loft") So as Ken says, get some extra. Two bags.

I used perlite and had about 30% compaction.
So with your 52x52x5, you have 7.8 cu ft.
If I were you I'd plan for 12 cu. ft - that allows for up to 40% compaction.
3 bags of vermiculite and 3 bags portland cement.
Jeff

Thanks for all the help. I went ahead and bought 6, 4 cuft. bags of medium vermiculite and 2, 92 pound bags of portland. I found the vermiculite cheap for $17.00 a bag and did not want to drive the 50 miles for it more than once. Ken was right and I can use the extra for insulating the dome. I'm sure that will take a lot.

I used up a little more than 2 bags of the vermiculite and a little more than a whole bag of the portland cement. Me and the wife mixed it in a wheel barrow and I shoveled it into the hearth. I screeded it off and had to cover it to keep the cats out of it. I'm sure they would love to make that a liter box.

Will this stuff take about a week to dry and cure before I can lay my firebrick and start my dome?

"Will this stuff take about a week to dry and cure before I can lay my firebrick and start my dome?"

It doesn't "dry", it cures. Like any portland cement based material, it must be kept well supplied with moisture for at least 7 days for it to obtain strength. That said, it will have enough compressive strength to continue your build as early as the next day. Lay your firebricks and start your dome. Just be a little gentle with it while it is still "green".

"Will this stuff take about a week to dry and cure before I can lay my firebrick and start my dome?"

It doesn't "dry", it cures. Like any portland cement based material, it must be kept well supplied with moisture for at least 7 days for it to obtain strength. That said, it will have enough compressive strength to continue your build as early as the next day. Lay your firebricks and start your dome. Just be a little gentle with it while it is still "green".

One other item. Keep your forms in place for a couple of weeks if you start the build right away. This will keep the material protected from chipping away until it reaches full strength.

No part of this website or content thereof may be reproduced, distributed or transmitted in any form or by any means, nor may any part of this website be stored in a database or other electronic retrieval system, or any other website, without the prior written permission of Forno Bravo.